You will have to "bit bang" 9 bits - SPI uses internal 8 bit register to output 8 bits.Or send out 16, first 7 as don't cares and the final 9 as the keepers, if your device will ignore them and just latch the last 9 on CS/SS going high.

Moderator Gammon asks what is the application for 9 bit SPI and asks for data sheets.

A common application is for graphic LCDs where the 9th bit is clear for a data byte and set for a command byte. Two displays using this are:http://adafruit.com/products/797 and http://adafruit.com/products/358

It would be a well received extension to the current library (documentation at http://arduino.cc/en/Reference/DueExtendedSPI) if this type of addressing could be accommodated for rather than a bitbanging method.

Several AVR libraries exist - they mostly turn off the hardware SPI, toggle the line manually if needed and clock, then turn it back on. This is very hard to do if one is using the SPI library, the functionality is not exposed.

If this cannot be accomplished, there is some code for bitbanging the SAM in a way similar to the AVR but it has not been ported to a tidy library. Tom Carpenter's gLCD libary appears to do this.